The present invention concerns a lighting circuit for HID lamps such as metal halides lamps used for illumination of indoor commercial facilities such as stores and outdoors facilities, light sources for liquid crystal projectors and headlights for use in automobiles or like other vehicles.
HID lamps (High Intensity Discharge Lamps) are also referred to as high luminance discharge lamps or high-pressure discharge lamps and since they are not only excellent in light emission efficiency relative to consumption power but also generate less amount of heat for an identical amount of light and have higher safety compared, for example, with halogen lamps, they have been used, in recent years, in a case where light sources of high luminance are required such as in illumination for indoor commercial facilities and outdoor facilities.
The HID lamp starts discharge by the application of a high voltage at about several kV upon starting and, continues discharge subsequently by applying a relatively low lamp voltage of several tens to several hundreds volts and the HID lamps is put in a lighted state along with increasing lamp voltage.
FIG. 9 shows a general light circuit 41 of lighting an HID lamp by an AC rectangular wave voltage and it comprises a main circuit 2 for applying a lamp voltage at several tens to several hundreds volts to an HID lamp 1, and a starting circuit 3 for applying a high starting voltage at several kilo volts.
The main circuit 2 comprises a rectifier circuit 5 for full wave rectification of a sinusoidal AC wave supplied from an AC power source 4, a power factor improving circuit 6 for converting a rectified pulsative voltage into a smooth DC voltage, a power control circuit comprising a chopper circuit 7A for converting the smooth DC voltage into rectangular pulses of a predetermined pulse width and a smoothing circuit 7B for smoothing the rectangular pulses again into a DC lamp voltage at a predetermined voltage value, and an inverter 9 for converting the obtained DC lamp voltage into an AC rectangular wave voltage at a voltage identical therewith, and the inverter 9 is connected by way of the starting circuit 3 to the HID lamp 1.
FIG. 10(a) shows a waveform of a lamp voltage V3 inputted to the inverter 9, and FIG. 10(b) shows a waveform of an AC rectangular wave voltage V4 outputted from the inverter 9.
The starting circuit 3 has a step-up transformer (not illustrated), which generates a high starting voltage at about several kilo volts so as to start discharge between electrodes of the HID lamp 1 when a lighting switch (not illustrated) of the HID lamp 1 is turned on.
Accordingly, in the lighting circuit 41, when the lighting switch (not illustrated) is turned on, a starting voltage at several kilo volts is applied to the HID lamp 1 to start discharge and, subsequent to the start of the discharge, discharge continues by the application of a relatively low lamp voltage of several tens to several hundreds volts supplied from the main circuit 2 and the lamp voltage increases gradually to put the HID lamp into a lighted state.
However, in a case of using the lighting circuit 41 described above, while there are no troubles where the distance of a wiring from the lighting circuit 41 to the HID lamp 1 is as short as within 2 m, erroneous operation may possibly be caused to the lighting circuit 41 as the distance increases.
Particularly, in a large retail store having a large shopping area per one floor such as a department store or supermarket, it is highly demanded to centralize various switches on one operation panel for enabling remote operation.
In such a case, for preventing erroneous operation of the lighting circuits 41, it is necessary that only the switches are located on the operation panel, while respective ignition circuits 41 are located near the respective HID lamps 1. This makes the wiring operation troublesome and makes the repair upon failure of the lighting circuit 41 also troublesome.
Accordingly, as a result of experiments and studies for analyzing the cause for the erroneous operation in the lighting circuit 41, it has been found that the inductance of wirings is not negligible when the wiring length increases, magnetic field energy is accumulated to the inductance of the wirings when the AC rectangular wave voltage is supplied from the main circuit 2 to the HID lamp 1, which causes counter-electromotive force on every inversion of the polarity of the voltage to reduce the voltage and the current to zero.
Then, the current forms an inverted spike waveform as shown in FIG. 10(c) by the counter-electromotive force, and the lamp voltage is increased instantaneously for recovery to form a spiked waveform as shown in FIG. 10(d), which causes erroneous operation of the lighting circuit 41.
For example, in a case of detecting a voltage applied to the main circuit 2 intending to control the lamp circuit, if the peak voltage of the spike wave should be detected, since a voltage abnormally higher than usual is detected, power control can not be conducted exactly.
On the other hand, it has been found during the experiment that the HID lamp 1 flickers in a case where the wirings are relatively short and spike waves described above are not formed.
This can not be considered as the erroneous operation of the lighting circuit 41, and the analysis for the cause has revealed that this is attributable to the characteristics of the HID lamp 1.
That is, it has been found that this dues to the difference of the resistance value, in a case of discharging the HID lamp 1 under inversion of polarity between electrodes, between a case of discharging from one electrode to the other electrode (positive direction) and a case of discharging from the other electrode to the one electrode (negative direction) even when structural characteristics and electrical characteristics of the electrodes are quite identical.
Then, it has been confirmed that when the voltage is applied in the direction of a larger resistance value, the position of an arc spot on the electrode is not stable but tends to be displaced to fluctuate the arc between the electrodes and cause flickering.
In view of the above, it is a technical subject of the present invention to prevent the lighting circuit from erroneous operation if counter-electromotive force should occur by magnetic field energy accumulated in the inductance of wirings in a case where a lighting circuit and an HID lamp are located at a long distance, and enable lighting with less flicker even when the inter-electrode resistance of the HID lamp is different depending on the polarity.
This invention provides a lighting circuit for an HID lamp in which a DC voltage supplied to a full-bridge type inverter having four switching elements is converted into an AC rectangular wave voltage at a predetermined frequency and applied to the HID lamp, comprising a control section for detecting a value of a current flowing in the HID lamp, calculating a resistance value in a positive direction and a resistance value in a negative direction of the lamp corresponding to the direction of the polarity of the AC rectangular wave voltage based thereon, setting a duty ratio between the positive direction and the negative direction of the AC rectangular wave voltage so as to be in an inverse proportion with the magnitude for each of the resistance values, and outputting a polarity control signal that switches each of the switching elements at a predetermined timing such that the voltage value is kept at a ground potential for a predetermined period of time when the direction of the AC rectangular waveform voltage is inverted.
According to the present invention, since the full bridge type inverter is used, the inputted constant DC voltage is converted into an AC rectangular wave voltage at a predetermined frequency by supplying polarity control signals so as to turn switching elements of each pair ON and OFF alternately, with two switching elements positioned orthogonally to each other being as a pair.
In this step, the value of the current flowing in the HID lamp is detected, the resistance value in the positive direction and the resistance value in the negative direction of the lamp corresponding to the polarity direction of the AC rectangular wave voltage are calculated based thereon, and a control signal is outputted such that the duty ratio between the positive direction and the negative direction of the AC rectangular wave voltage is in an inverse proportion with the magnitude of each resistance value.
Thus, for the duty ratio in the positive direction and the negative direction of the AC rectangular wave voltage, the polarity with the higher resistance value is decreases while the polarity with the lower resistance value decreases.
Accordingly, the period of time during which a voltage with a polarity of larger resistance value is shortened to cause less flicker by fluctuation.
Further, by switching the switching elements of each pair ON and OFF at a predetermined timing, an interval of keeping the voltage value to the ground potential only for a predetermined period of time (for example {fraction (1/10)} of one period) is formed when the direction of the AC rectangular wave voltage is inverted.
Accordingly, while the counter-electromotive force tending to keep the voltage so far is generated at the instance the AC rectangular wave voltage outputted from the inverter is reduced to the earth potential, since the current is released as it is to the ground, it does not form a spike wave that would cause erroneous operation in the lighting circuit.